<p>The evaluation of deformation properties in rock masses under simple-shear mode is critical, particularly for assessing rock behavior during seismic events. Currently, there is no standardized method in Japan for simple-shear testing of rocks. Additionally, precise identification of mechanical properties required for evaluating seismic stability of rock masses and the stiffness of basement rocks for seismic response analysis remains elusive. This study introduces a novel simple-shear test for prismatic rock specimens, utilizing a torsional-shear testing apparatus. The method involves torsionally shearing a prismatic specimen by applying both normal load and torque, which is radially positioned on the pedestal of the apparatus. We conducted experiments using Japanese tuff prism specimens of various depths to examine the effect of radial load distribution induced by torque on the stress–strain relationship. Results from tests conducted under a 2&#xa0;MPa normal stress condition indicate that shorter specimens demonstrate more uniform stress–strain curves, attributed to even tensile failure across the radial orientation. In contrast, longer specimens displayed complex failure modes due to progressive failure mechanisms. In addition, analysis of the stress states in the specimens revealed that shorter specimens yielded Mohr’s stress circles that align with failure criteria established in other tests. These findings confirm the effectiveness of the proposed testing method and suggest optimal dimensions for the specimens.</p>